A hard real-time application within a data network is a type of data communication in which a message is transmitted and successfully received over the network prior to a certain known deadline and with sufficient degree of certainty. An example of this would be control of a wing flap over an avionics bus. Thus, a minimum data rate and a maximum packet error rate must be guaranteed, under all operating conditions of the network. For multiple hard real-time applications, aggregate minimum data rate and maximum packet error rate requirements guaranteeing performance of the applications under all operating conditions can be defined. The set of these aggregate rates for a given set of hard real-time applications will be referred herein as the associated set of predefined network requirements (PNRs).
Similarly, a non-hard real-time application refers to a data communication process where there is no real-time constraint or deadline.
Current network performance of a given network is defined herein as a metric indicative of the data rate and bit error rate for each node pair in the network, at a given point in time. It is assumed to be substantially constant from the moment it is measured over a given period of time, following which a new current network performance measurement may be required.
Hard real time systems can be constructed using different media access protocols including a command/response protocol or a priority based protocol. Among others, the MIL-STD-1553 or simply 1553 is generally utilized for hard real time communication. 1553 is an approximately 30 year old technology that defines the electrical and signaling characteristics for 1 Mbps data rate communications over an asynchronous serial, command/response digital data bus on which messages are time division multiplexed among users. The United States Department of Defense (“DoD”) requires the use of 1553 as the standard for all inter and intra-subsystem communications on all military airplanes, helicopters, ships and land vehicles. Originally used only in mission avionics, 1553 is now used in flight critical avionics, flight control, weapons, electrical power control, and propulsion control. 1553 specifies all of the electrical characteristics of the receivers, transmitters, and cable used to implement the bus, as well as the complete message transmission protocol. The messages are generally highly repetitive, and their content and periodicity are all pre-planned. The data capacity of 1 Mbps also comes with an associated bit error rate. Various notices, such as 1553a, 1553b, up to 1553e of the standard are available. For 1553b, there is a requirement to have a word or packet error rate of 1 in 10,000,000. In a lot of cases the bus will in fact support much higher communications rates at the same or lower packet error rate.
MIL-STD-1553B utilizes a primitive Manchester II bi-phase signaling scheme over shielded twisted pair cabling. This modulation scheme is bandwidth inefficient with most of its signal energy concentrated around 1 MHz. MIL-STD-1553b has little remaining capacity for existing applications and leaves little opportunity to enable additional communication capabilities.
Notice 5 of MIL-STD-1553 or 1553e provides an overlap of 1553 signals with signals based on Orthogonal Frequency Division Multiplexing (OFDM), for increased bandwidth availability for additional applications and without impact to the existent 1553 communication.
OFDM communication systems are generally designed to be rate adaptive in order to take advantage of the higher communications capacity of a particular channel at a particular time. Examples include technologies such as: 802.11a, HomePlug, HomePNA, WiMax, etc. Rate adaptation is usually implemented based on metric calculations from exchanging signals between two or more nodes on a network. These metrics are generally related to the signal to noise ratio (SNR) at the receiving device with the transmitter agreeing to use suitable modulation and coding to maximize the throughput for particular bit error rate for the channel. For OFDM communications, the number of bits assigned to an OFDM sub-carrier can be adapted independently for all sub-carriers.
A managed network (like a 1553b avionics network) refers to a controlled network where a network designer would configure and specify the number of communications devices as well as the details of the communications taking place. For 1553b, every message that is transmitted is specified according to a schedule delivered by a bus controller.